1. Field of the Invention
The present invention relates to a vertical contour correction device in a picture imaging apparatus for color television receivers and the like, and in particular to a vertical contour correction device adapted to give simultaneously preshoots and overhoots to the contour portion in the vertical direction.
2. Description of the Prior Art
The bandwidth of the video signals of the transmitting and receiving system for television is generally limited 4.2 MHz. Signal components in the higher frequency band than this are not transmitted so that there exist in principle such phenomenon as rounding in fine portions or edge portions of the original pattern, constituting the main factor for the degradation of the sharpness of the television pictures produced.
To improve the sharpness, there has been known a technique of adding preshoots and overshoots in the front and rear of the edge portion of the video signal. This will be described by referring to FIGS. 1a to 1c.
When an image signal which varies in step fashion as shown in FIG. 1a is inputted, a contour correction signal can be obtained by taking the first and second derivatives of the signal. FIG. 2b shows an example of the twice differentiated waveform. Contour correction is accomplished by obtaining a signal with a preshoot and an overshoot, as shown in FIG. 1c, through superposition of the wave-form signal of FIG. 1(b) to the input signal of FIG. 1(a).
However, the contour correction like in the above has the following problem. Namely, to obtain the twice-differentiated waveform a circuit which includes an inductor, a capacitor, and a resistor is usually used. This means basically that contour correction can be achieved for the horizontal direction alone of the reproduced picture.
In a transmitting and receiving system for television, the grayishness of the picture element is converted to an electrical signal continuous in time by scanning the picture on the transmitting side of the picture. The signal is reproduced on the receiving side of the picture as a two-dimensional picture by carrying out assembly scanning synchronous with the decomposition scanning of the picture on the transmitting side. Because of this, the signal at the time earlier by one horizontal scanning period (1H) is needed in order to give a contour correction for the vertical direction of the picture.
Signals at a time earlier by 1H may be obtained by the use of a delay circuit like a charge coupled device (CCD), and as a prior art device for accomplishing vertical contour correction by means of such a delay circuit there has been known one like the following.
Namely, the prior art vertical contour correction device comprises a 1H delay circuit (referred to as 1H delay line hereafter) of a CCD or the like connected to the input terminal of the video signal, a subtraction circuit for subtracting the output signal of the 1H delay line from the video signal, an amplifier for adjusting the amplitude of the output signal of the subtraction signal with a prescribed amplification factor, and an addition circuit for outputting the result of adding the signal from the amplifier to the input video signal.
With a correction circuit of the above construction, it is possible to obtain an output signal waveform with overshoot added on at the leading edge and the trailing edge in the vertical direction.
It is noted that, although a correction circuit which adds overshoots is described in the foregoing, a circuit which is to add preshoots could also be readily inferred.
However, such a correction where only one of overshoot or preshoot is added is not quite satisfactory because of the unnatural result of the reproduced picture, though admittedly it improves the sharpness to some degree. For this reason, it has been desired to add the overshoot and the preshoot at the same time.
To resolve this problem, there has been provided a vertical contour correction device which is adapted to give the preshoot and the overshoot at the same time. This prior art vertical contour correction device comprises a first 1H delay line connected to a input terminal of the video signal, a second 1H delay line connected to the output terminal of the first 1H delay line, a first inverting amplifier for inverting and amplifying the input video signal, an amplifier for amplifying the output signal of the first 1H delay line, a second inverting amplifier for inverting and amplifying the output signal of the second 1H delay line, and an adder for outputting the sum of the output signals of the first and second inverting amplifiers and the amplifier.
By the use of vertical contour correction device like the above, it is possible to obtain an output signal waveform which has preshoots and overshoots at the vertical leading edge and the vertical trailing edge.
However, a vertical contour correction device like the above creates a new disadvantage in that it requires two 1H delay lines so that the circuit involved becomes large when constructed with a delay circuit like a CCD.